This seems to return Feb 22nd 2014 though. So it seems to differ from MS Excel's. If you want to be able to interchange between Excel and AHK, you'll need to use Excel's formula for calculating it and figure out where the discrepancy lies.

You need to subtract 2 from the days; 1 because you start on January 1st, rather than January "0th", and 1 because of a bug dating all the way back to Lotus 1-2-3, detailed here: https://support.microsoft.com/en-us/kb/214326 (basically, that program assumed 1990 was a leap year, but it wasn't, and for compatibility MS kept that "bug" intentionally in Excel).

EDIT: Alternative solution is to decrease the start date by two days i.e. start:=18991230000000. Both are equivalent and it makes little to no difference which you use. I guess changing the start value would be technically slightly more efficient as you have one less calculation, but you'd have to be doing a massive amount of iterations for it to make any difference I'm quite sure.